Use template_redirect and post_type_link filters in WordPress to remove the CPT slug from the front of CPT URLs.

my_cpt_404_redirects.php

/*
	If 404 on a post, look for a gallery or portfolio with the same slug.
	
	Borrows from: https://wordpress.stackexchange.com/a/45081/3652
*/
function my_cpt_404_redirects($template)
{	
	if(is_404())
	{
		//see if we know the exact post ID
		$id = max(get_query_var('p'), get_query_var('page_id'), get_query_var('attachment_id'));				
		$redirect_url = false;
		if ($id && $redirect_post = get_post($id)) {
			$post_type_obj = get_post_type_object($redirect_post->post_type);
			if ($post_type_obj->public)
				$redirect_url = get_permalink($redirect_post);
		}
		
		//if not, let's see if there is a gallery or portfolio with the same slug and date
		if (!$redirect_url)
		{
			global $wp_query, $post, $post_type, $wpdb;
			$sqlQuery = "SELECT ID FROM $wpdb->posts WHERE post_type IN('galleries', 'portfolios') AND post_status = 'publish' AND YEAR(post_date) = '" . esc_sql($wp_query->query_vars['year']) . "' AND MONTH(post_date) = '" . esc_sql($wp_query->query_vars['monthnum']) . "' AND post_name = '" . esc_sql($wp_query->query_vars['name']) . "' LIMIT 1";			
			$post_id = $wpdb->get_var($sqlQuery);
			
			if(!empty($post_id))
			{			
				//found one!
				
				//reset the query
				$wp_query = new WP_Query('p=' . $post_id);
				
				//reset global $post
				$post = get_post($post_id);
				$post_type = $post->post_type;
				setup_postdata( $post->ID);
								
				//don't redirect to the canonical URL
				remove_filter('template_redirect', 'redirect_canonical');
				//reset 404 to 200 status
				status_header(200);
			}
		}	
	}
	else
	{
		/*
		//debug
		global $wp_query;		
		d($wp_query);
		*/
	}
	
	return $template;
}
add_action('template_redirect', 'my_cpt_404_redirects', 5);

/*
	Change permalinks for portfolios and galleries to use year/month/slug
*/
function my_post_link($url, $post, $leavename)
{
	$status = $post->post_status;
	$type = $post->post_type;
	
	if($status == "publish" && ($type == "galleries" || $type == "portfolios"))
	{
		$unixtime = strtotime($post->post_date);
		$date = explode(" ",date('Y m d H i s', $unixtime));

		$url = home_url($date[0] . "/" . $date[1] . "/" . $post->post_name . "/");
	}

	return $url;
}
add_filter('post_type_link', 'my_post_link', 10, 3);